U.S. patent application number 12/050661 was filed with the patent office on 2008-09-25 for emission control in aged active matrix oled display using voltage ratio or current ratio.
This patent application is currently assigned to Leadis Technology, Inc.. Invention is credited to William Robert Bidermann, Walter Edward Naugler.
Application Number | 20080231557 12/050661 |
Document ID | / |
Family ID | 39766433 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080231557 |
Kind Code |
A1 |
Naugler; Walter Edward ; et
al. |
September 25, 2008 |
EMISSION CONTROL IN AGED ACTIVE MATRIX OLED DISPLAY USING VOLTAGE
RATIO OR CURRENT RATIO
Abstract
Compensation needed to be made for reduced light efficiency in
aged sub-pixels of an active matrix organic light-emitting diode
(OLED) display are determined using a current ratio or a voltage
ratio pertaining to an aged sub-pixel relative to un-aged,
reference sub-pixels.
Inventors: |
Naugler; Walter Edward;
(Katy, TX) ; Bidermann; William Robert; (San
Diego, CA) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER, 801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Assignee: |
Leadis Technology, Inc.
Sunnyvale
CA
|
Family ID: |
39766433 |
Appl. No.: |
12/050661 |
Filed: |
March 18, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60919195 |
Mar 20, 2007 |
|
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|
60919228 |
Mar 20, 2007 |
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Current U.S.
Class: |
345/76 |
Current CPC
Class: |
G09G 3/3291 20130101;
G09G 2300/0842 20130101; G09G 2320/0233 20130101; G09G 2320/0693
20130101; G09G 3/3225 20130101; G09G 2320/0276 20130101; G09G
2320/029 20130101; G09G 2320/048 20130101 |
Class at
Publication: |
345/76 |
International
Class: |
G09G 3/30 20060101
G09G003/30 |
Claims
1. A method of determining compensation needed for reduced light
efficiency in aged sub-pixels of an active matrix organic
light-emitting diode (OLED) display, the method comprising: aging a
plurality of sections of sub-pixels of a first active matrix OLED
display, the sections including at least a first section including
aged sub-pixels and a second section including reference sub-pixels
that are not aged; applying a predetermined voltage across one or
more of the aged sub-pixels and one or more of the reference
sub-pixels; determining a first current through said one or more of
the aged sub-pixels and a second current through said one or more
of the reference sub-pixels; determining an age of said one or more
of the aged sub-pixels based on the first current relative to the
second current; determining light emission characteristics of said
one or more of the aged sub-pixels; and determining corrections to
be made to digital numbers indicative of desired brightness in said
one or more of the aged sub-pixels based on mappings between the
determined age and the determined light emission characteristics of
said one or more of the aged sub-pixels.
2. The method of claim 1, wherein the plurality of sections of the
sub-pixels are aged for a same predetermined period of time but
with different amounts of current flowing through two or more of
the sections of the sub-pixels to obtain different effective ages
in said two or more of the sections of the sub-pixels of the OLED
display.
3. The method of claim 1, wherein determining the first current
includes measuring the first current through two or more of the
aged sub-pixels and averaging the measured first current, and
determining the second current includes measuring the second
current through two or more of the reference sub-pixels and
averaging the measured second current.
4. The method of claim 1, wherein each of the sub-pixels of the
active matrix OLED display include a thin film transistor
configured to drive an OLED of the sub-pixel, and current through
the aged sub-pixel or the reference sub-pixel is measured with the
thin film transistor biased in linear mode.
5. The method of claim 1, wherein the age of said one or more of
the aged sub-pixels is determined based on a current ratio of the
first current to the second current, the current ratio being less
than one and being smaller as the sub-pixels have longer effective
age.
6. The method of claim 5, further comprising storing a mapping
between the current ratio and the age of the aged sub-pixel in a
selection look-up table.
7. The method of claim 1, further comprising storing said
corrections to be made to the digital numbers in one of a plurality
of age curve look-up tables corresponding to the determined age of
the aged sub-pixel, each age curve look-up table corresponding to a
different age of the aged sub-pixel and mapping the digital numbers
to said corrections to be made to the digital numbers for the
corresponding age of the aged sub-pixel, wherein one or more of the
aged sub-pixels of the OLED display are assigned to use said one of
the age curve look-up tables for correction of the digital
numbers.
8. The method of claim 7, further comprising: after a second active
matrix OLED display is aged by actual use, applying the
predetermined voltage across one of aged sub-pixels of the second
active matrix OLED display and at least one of reference sub-pixels
of the second active matrix OLED display; determining a third
current through said one of the aged sub-pixels of the second
active matrix OLED display and a fourth current through said at
least one of the reference sub-pixels of the second active matrix
OLED display; determining an age of said one of the aged sub-pixels
of the second active matrix OLED display based on the third current
relative to the fourth current; selecting one of the age curve
look-up tables to use for correction of the digital numbers
indicative of desired brightness in said one of the aged sub-pixels
of the second active matrix OLED display, based upon the determined
age of said one of the aged sub-pixels of the second active matrix
OLED display.
9. The method of claim 8, further comprising storing mappings
between a sub-pixel number corresponding to said one of the aged
sub-pixels of the second active matrix OLED display and an age
curve look-up table number corresponding to the selected age curve
look-up table in a correction look-up table.
10. A method of determining compensation needed for reduced light
efficiency in aged sub-pixels of an active matrix organic
light-emitting diode (OLED) display, the method comprising: aging a
plurality of sections of sub-pixels of a first active matrix OLED
display, the sections including at least a first section including
aged sub-pixels and a second section including reference sub-pixels
that are not aged; determining a first voltage applied to one or
more of the aged sub-pixels to generate a predetermined reference
current through said one or more of the aged sub-pixels;
determining a second voltage applied to one or more of the
reference sub-pixels to generate the same predetermined reference
current through said one or more of the reference sub-pixels;
determining an age of said one or more of the aged sub-pixels based
on the first voltage relative to the second voltage; determining
light emission characteristics of said one or more of the aged
sub-pixels; determining corrections to be made to digital numbers
indicative of desired brightness in said one or more of the aged
sub-pixels based on mappings between the determined age and the
determined light emission characteristics of said one or more of
the aged sub-pixels.
11. The method of claim 10, wherein the plurality of sections of
the sub-pixels are aged for a same predetermined period of time but
with different amounts of current flowing through two or more of
the sections of the sub-pixels to obtain different effective ages
in said two or more of the sections of the sub-pixels of the OLED
display.
12. The method of claim 10, wherein determining the first voltage
includes measuring the first voltage across two or more of the aged
sub-pixels with the same predetermined reference current flowing
through said two or more of the aged sub-pixels and averaging the
measured first voltage, and determining the second voltage includes
measuring the second voltage across two or more of the reference
sub-pixels with the same predetermined reference current flowing
through said two or more of the reference sub-pixels and averaging
the measured second voltage.
13. The method of claim 10, wherein each of the sub-pixels of the
active matrix OLED display include a thin film transistor
configured to drive an OLED of the sub-pixel, and voltage across
the aged sub-pixel or the reference sub-pixel is measured with the
thin film transistor biased in linear mode.
14. The method of claim 10, wherein the age of said one or more of
the aged sub-pixels is determined based on a voltage ratio of the
first voltage to the second voltage, the voltage ratio being
greater than one and being larger as the sub-pixels have longer
effective age.
15. The method of claim 15, further comprising storing a mapping
between the voltage ratio and the age of the aged sub-pixel in a
selection look-up table.
16. The method of claim 10, further comprising storing said
corrections to be made to the digital numbers in one of a plurality
of age curve look-up tables corresponding to the determined age of
the aged sub-pixel, each age curve look-up table corresponding to a
different age of the aged sub-pixel and mapping the digital numbers
to said corrections to be made to the digital numbers for the
corresponding age of the aged sub-pixel, wherein one or more of the
aged sub-pixels of the OLED display are assigned to use said one of
the age curve look-up tables for correction of the digital
numbers.
17. The method of claim 16, further comprising: after a second
active matrix OLED display is aged by actual use, determining a
third voltage applied to one of aged sub-pixels of the second
active matrix OLED display to generate the predetermined reference
current through one of the aged sub-pixels of the second active
matrix OLED display, and determining a fourth voltage applied to
one or more of reference sub-pixels of the second active matrix
OLED display to generate the same predetermined reference current
through said one or more of the reference sub-pixels of the second
active matrix OLED display; determining an age of said one of the
aged sub-pixels of the second active matrix OLED display based on
the third voltage relative to the fourth voltage; and selecting one
of the age curve look-up tables to use for correction of the
digital numbers indicative of desired brightness in said one of the
aged sub-pixels of the second active matrix OLED display, based
upon the determined age of said one of the aged sub-pixels of the
second active matrix OLED display.
18. The method of claim 17, further comprising storing mappings
between a sub-pixel number corresponding to said one of the aged
sub-pixels of the second active matrix OLED display and an age
curve look-up table number corresponding to the selected age curve
look-up table in a correction look-up table.
19. An active matrix organic light-emitting diode (OLED) display
comprising: a plurality of OLED elements arranged in a plurality of
rows and a plurality of columns, each of the OLED elements
corresponding to a sub-pixel of the OLED display; and an active
matrix drive circuit configured to drive current through the OLED
elements, the active matrix drive circuit including: a plurality of
age curve look-up tables each corresponding to a different age of
aged sub-pixels of the OLED display and mapping digital numbers to
corrections to be made to the digital numbers for the corresponding
age of the aged sub-pixel, one or more of the aged sub-pixels of
the OLED display being assigned to use said one of the age curve
look-up tables for correction of the digital numbers; a correction
look-up table storing mappings between each of the OLED sub-pixels
and said assigned one of the age curve look-up tables; and a
selection look-up table storing mappings between ages of the
sub-pixels of the OLED display and the age curve look-up tables,
the ages of the sub-pixels represented by current ratios or voltage
ratios associated with the sub-pixels of the OLED display, and
wherein the active matrix drive circuit is configured to receive
the digital number indicative of a desired brightness of one of the
aged sub-pixels and generate a corrected digital number for driving
the OLED elements using the corrections to the digital numbers
stored in the age curve look-up table assigned to said one of the
aged sub-pixels.
20. The active matrix organic light-emitting diode (OLED) display
of claim 19, wherein the active matrix drive circuit further
comprises a calibration engine configured to: apply a predetermined
voltage across said one of the aged sub-pixels and at least one of
the reference sub-pixels; determine a first current through said
one of the aged sub-pixels and a second current through said at
least one of the reference sub-pixels; determine an age of said one
of the aged sub-pixels based on the first current relative to the
second current; and select one of the age curve look-up tables to
use for correction of the digital numbers in said one of the aged
sub-pixels for storage in the correction look-up table, based upon
the determined age of said one of the aged sub-pixels.
21. The active matrix organic light-emitting diode (OLED) display
of claim 19, wherein the active matrix drive circuit further
comprises a calibration engine configured to: determine a first
voltage applied to said one of the aged sub-pixels to generate a
predetermined reference current through said one of the aged
sub-pixels, and determine a second voltage applied to one or more
of reference sub-pixels that are un-aged to generate the same
predetermined reference current through said one or more of the
reference sub-pixels; determine an age of said one of the aged
sub-pixels based on the first voltage relative to the second
voltage; and select one of the age curve look-up tables to use for
correction to the digital numbers in said one of the aged sub-pixel
for storage in the correction look-up table, based upon the
determined age of said one of the aged sub-pixels.
22. The active matrix organic light-emitting diode (OLED) display
of claim 19, wherein the age curve look-up tables store mappings
between the digital numbers and increases or decreases to be made
to the digital numbers.
23. The active matrix organic light-emitting diode (OLED) display
of claim 19, wherein the age curve look-up tables store mappings
between the digital numbers and corrected digital numbers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) from (i) co-pending U.S. Provisional Patent
Application No. 60/919,195 entitled "Method for Emission Control
for Pixels in an Active Matrix Emissive Display Using Current
Ratios," filed on Mar. 20, 2007 and (ii) co-pending U.S.
Provisional Patent Application No. 60/919,228 entitled "Method for
Emission Control for Pixels in an Active Matrix Emissive Display
Using Voltage Ratios," filed on Mar. 20, 2007, both of which are
incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to modifying the current fed
to an aging OLED sub-pixel in order to maintain constant light
emission at a desired gray level.
[0004] 2. Description of the Related Arts
[0005] An OLED display is generally comprised of an array of
organic light emitting diodes (OLEDs) that have carbon-based films
disposed between two charged electrodes. Generally one electrode is
comprised of a transparent conductor, for example, indium tin oxide
(ITO). Generally, the organic material films are comprised of a
hole-injection layer, a hole-transport layer, an emissive layer and
an electron-transport layer. When voltage is applied to the OLED,
the injected positive and negative charges recombine in the
emissive layer and transduce electrical energy to light energy.
Unlike liquid crystal displays (LCDs) that require backlighting,
OLED displays are self-emissive devices--they emit light rather
than modulate transmitted or reflected light.
[0006] An OLED display typically includes a plurality of OLEDs
arranged in a matrix form including a plurality of rows and a
plurality of columns, with the intersection of each row and each
column forming a pixel of the OLED display. An OLED display is
generally activated by way of a current driving method that relies
on either a passive-matrix (PM) scheme or an active-matrix (AM)
scheme.
[0007] In a passive matrix OLED display, a matrix of
electrically-conducting rows and columns forms a two-dimensional
array of picture elements called pixels. Sandwiched between the
orthogonal column and row lines are thin films of organic material
of the OLEDs that are activated to emit light when current is
applied to the designated row and column lines. The brightness of
each pixel is proportional to the amount of current applied to the
OLED of the pixel. While PMOLEDs are fairly simple structures to
design and fabricate, they demand relatively expensive,
current-sourced drive electronics to operate effectively and are
limited as to the number of lines because only one line can be on
at a time and therefore the PMOLED must have instantaneous
brightness equal to the desired average brightness times the number
of lines. Thus, PMOLED displays are typically limited to under 100
lines. In addition, their power consumption is significantly higher
than that required by an active-matrix OLED. PMOLED displays are
most practical in alpha-numeric displays rather than higher
resolution graphic displays.
[0008] An active-matrix OLED (AMOLED) display is comprised of OLED
pixels that have been deposited or integrated onto a thin film
transistor (TFT) array to form a matrix of pixels that emit light
upon electrical activation. In contrast to a PMOLED display, where
electricity is distributed row by row, the active-matrix TFT
backplane acts as an array of switches coupled with sample and hold
circuitry that control and hold the amount of current flowing
through each individual OLED pixel during the total frame time. The
active matrix TFT array continuously controls the current that
flows to the OLEDs in the each of pixels, signaling to each OLED
how brightly to illuminate.
[0009] FIG. 1 illustrates a conventional active matrix OLED
display. While the example of FIG. 1 is illustrated as an OLED
display, other emissive-type displays would have structures similar
to that illustrated in FIG. 1. Referring to FIG. 1, the OLED
display panel includes a plurality of rows Row 1, Row 2, . . . ,
Row Y and a plurality of columns Col. 1, Col. 2, . . . , Col. X
arranged in a matrix. The intersection of each row and each column
forms a pixel of the OLED display. The OLED display also includes a
Gamma network 104, row drivers 116-1, 116-2, . . . , 116-y, column
drivers 114-1, 114-2, . . . , 114-x, and a timing controller
112.
[0010] For a color OLED display, each pixel includes 3 sub-pixels
that have similar structure but emit different colors (R, G, B).
For simplicity of illustration, FIG. 1 illustrates only one
sub-pixel (denoted as dashed line boxes in FIG. 1, such as box 120)
corresponding to one of the R, G, B colors per pixel at the
intersection of each row and each column. However, in real OLED
display panels, each pixel includes three identical ones of the
sub-pixel structure 120 as illustrated in FIG. 1. As shown in FIG.
1, the active drive circuitry of each sub-pixel 120 includes TFTs
T1 and T2 and a storage capacitor Cs for driving the OLED D1 of the
sub-pixel 120. In the following explanation of FIG. 1, the type of
the TFTs T1 and T2 is a p-channel TFT. However, note that n-channel
TFTs may also be utilized in the active matrix.
[0011] Image data 110 includes data indicating which sub-pixel 120
of the OLED display should be turned on and the brightness of each
sub-pixel. Image data 110 is sent by an image rendering device
(e.g., graphics controller (not shown herein)) to the timing
controller 112, which coordinates column and row timing. The timing
controller 112 sends digital numbers (DN) 101 indicating pixel
brightness to the gamma network 104. Row timing data 105 included
in image data 110 is coupled to the gate lines 150 of each row
through its corresponding row driver 116-1, 116-2, . . . , 116-y.
Row drivers 116-1, 116-2, . . . , 116-y drive the gate line 150 so
that the gate lines 150 carry a voltage of 25 to 30 volts when
active. The gates of TFTs T2 of each sub-pixel in a row are
connected to gate line 150 of each row to enable TFTs T2 to operate
as switches. The data lines 160 are connected to the sources of
TFTs T2 in each column. When the gate line 150 becomes active for a
row based on the row timing data 105, all the TFTs T2 in the row
are turned on. Timing controller 112 sends column timing data 106
to the column drivers 114-1, 114-2, . . . , 114-x. The Gamma
network 104 generates the T1 gate voltages 102 (brightness) to be
applied to each TFT T1 in the row when the sub-pixel 120 is turned
on, based on digital numbers (DNs) 101 corresponding to each gate
voltage 102. Column drivers 114-1, 114-2, . . . , 114-x provides
analog voltages 160 to be applied to the gates of TFTs T1,
corresponding to the T1 gate voltages 102. The voltages 102
representing pixel brightness values are distributed from the Gamma
network 104 to all the column drivers 114-1, 114-2, . . . , 114-x
in parallel after the appropriate T1 gate voltages 102 have been
sent from gamma network 104 to each column driver 114-1, 114-2, . .
. , 114-x under control of the column timing data 106 from timing
controller 112. Under control of the timing controller 112, for
example, row driver 1 (116-1) is activated and all the voltages 102
placed on the column drivers 114-1, 114-2, . . . , 114-x are
downloaded to the TFT T1s in row 1. Timing controller 112 then
proceeds to send brightness data for the next row (e.g., row 2)
using the row driver 2 (116-2) to column drivers 114-1 through
114-x and activating row 2 and so forth, until all rows have been
activated and brightness data for the total frame has been
downloaded and all the sub-pixels are turned on to the brightness
indicated by the image data 110.
[0012] The drain of TFT T2 is connected to the gate of TFT T1 and
to one side of storage capacitor Cs. The source of TFT T1 is
connected to positive supply voltage VDD. The other side of storage
capacitor Cs is also connected, for example, to the positive supply
voltage VDD and to the source of TFT T1. Note that the storage
capacitor Cs may be tied to any reference electrode in the pixel.
The drain of TFT T1 is connected to the anode of OLED D1. The
cathode of OLED D1 is connected to negative supply voltage Vss or
common Ground. The analog voltages 160 are downloaded to the OLED
display a row at a time.
[0013] When TFT T2 is turned on, the analog T1 gate voltage 160 is
applied to the gate of each TFT T1 of each sub-pixel 120, which is
locked by storage capacitor Cs. When the row scan moves to the next
row, the gate voltage of TFT T1 is locked for the frame time until
the next gate voltage for that sub-pixel is sent by the column
drivers 114-1, 114-2, . . . , 114-n. In other words, the continuous
current flow to the OLEDs is controlled by the two TFTs T1, T2 of
each sub-pixel. TFT T2 is used to start and stop the charging of
storage capacitor Cs, which provides a voltage source to the gate
of TFT T1 at the level needed to create a constant current to the
OLED D1. As a result, the AMOLED display operates at all times
(i.e., for the entire frame scan), avoiding the need for the very
high instantaneous currents required for passive matrix operation.
The TFT T2 samples the data on the data line 160, which is held as
charge stored in the storage capacitor Cs. The voltage held on the
storage capacitor Cs is applied to the gate of the second TFT T1.
In response, TFT T1 drives current through the OLED D1 to a
specific brightness depending on the value of the sampled and held
data signal as stored in the storage capacitor Cs.
[0014] FIG. 2 illustrates a conventional gamma network used with an
active matrix OLED display. The gamma network 104 is a circuit that
converts the brightness data for a sub-pixel from a digital number
(DN) representing the desired gray level (brightness) to an analog
voltage, which will produce the right amount of current to drive
OLED D1 to emit the desired brightness when the analog voltage 160
is applied to the gate of TFT T1 in the sub-pixel 120 (See FIG. 1).
For example, the gamma network 104 in FIG. 2 is a conventional 8
bit gamma network used with DN (8 bits) ranging from 0 to 255.
Gamma network 104 includes a counter 202, a decoder 204, a series
of resistors (R0, . . . , R30, . . . R191, . . . , R223, . . . ,
R253, R254) (255 resistors for an 8 bit system) and 256 switches
GT0, GT1, . . . , GT255. The gate of each switch GT0, GT1, . . . ,
GT255 is coupled to the corresponding one of the bits of decoder
204. When the corresponding binary bit at the decoder 204 is "1"
the corresponding switch (GT0, GT1, . . . , GT255) is turned on,
and when the binary bit at the decoder 204 is "0" the corresponding
switch (GT0, GT1, GT255) is turned off. DN 101 can be any value
between 0 and 255 for an eight bit system. Counter 202 counts up to
the value of DN 101 sent to the Gamma network 104, causing decoder
204 to move its output to the gate of the gamma table switches
GT(DN). For example, if a DN of 185 indicating brightness level 185
was sent to counter 202, decoder 204 would move its output to
GT185, thereby switching switch GT185 on. Gamma network 104 is
essentially a voltage divider with 256 taps corresponding to 256
gray levels (brightnesses). The voltage at tap 185 is controlled by
switch GT 185, which when turned on delivers to the gate of the TFT
T1 in the specified sup-pixel the voltage calculated to produce a
gray level brightness corresponding to DN 185.
[0015] The voltage 102 output from the gamma network 104 is
designed to produce a series of currents from TFT T1 that will
produce 256 levels (in an 8 bit display system) of light emission
from OLED D1 conforming to the brightness response of the human
eye. The human eye is logarithmically sensitive to brightness and
thus approximately has a linear response approximate to the square
of brightness. That is, for the human eye to experience a doubling
of brightness, the light flux has to be increased approximately 4
times. This relationship of eye response to light flux (brightness)
is known as the gamma function (y), which is not exactly 2 but
closer to 2.2. In general, gamma gives contrast to the image. If,
for example, gamma is reduced to 1 (a linear relationship between
eye response and light), the images produced would have very low
contrast, and be flat and very uninteresting. If gamma is
increased, contrast of the image increases. Note that gamma refers
to the relationship between the eye and light--not current or
voltages. OLED emission is produced by current flowing through OLED
D1 as controlled by TFT T1. Thus, it is the function of the gamma
network 104 to produce an appropriate voltage, which will produce
appropriate current through OLED D1, which will produce light with
the correct (or desired) gamma function. The emission of light from
OLED material is linear to the current. That is, in order to double
the luminance (expressed as cd/m.sup.2--candelas per meter
squared), current is doubled.
[0016] The brightness values in an image are represented as digital
numbers (DNs). For an 8-bit display system, DNs range from 0 to
255. The light values are called gray scale levels and are linear
to the human eye. Thus, a doubling of DNs is perceived by the human
eye as a doubling of brightness. The gamma relation between DNs and
the current of TFT T1 can be determined as follows. FIG. 3A
illustrates the gamma curve showing the relationship between the
digital number (DN) and the OLED current. Note that gamma curve 300
is not linear but has a curve with a changing slope. The exact
shape of the gamma curve 300 is determined by the desired gamma.
The gamma curve 300 shown in FIG. 3A is for a gamma of 2.
[0017] FIG. 3B is a table showing example resistors, voltages and
currents for the gamma network in FIG. 2. Referring to FIGS. 2 and
3B, note that the resistors (R0 through R254) are grouped with
roughly 32 resistors per group, except Group 0 that includes no
resistor, although all the resistors are not shown in FIG. 2 for
simplicity of illustration. Each resistor group (Group 0 through
Group 8) is associated with a tap voltage Vtap0 through Vtap7 and
Vgamma. The tap voltages, for example, are bounded by a minimum
voltage (1.541 volts) and a maximum voltage (Vgamma, 12.000 volts).
The tap voltages coupled with the minimum and maximum voltages
establish the gamma current curve 300 with the aid of resistors R0
through R254. The tap voltages are voltage sources, and thus the
voltage established between each resistor is determined by the
current drawn between the tap voltages. The greater the number of
tap voltages, the better current conformation is to the gamma
curve. In the example of FIG. 3B, nine voltage sources produce the
voltages at each resistor (R0 through R254), which in turn use TFT
T1 to produce the current that conforms to the gamma curve 300. By
adjusting the tap voltages, the gamma current curve 300 will
change.
[0018] The gate voltage 102 to the TFT T1 is determined by the tap
voltages, resistors, and which of the switches GT0, . . . , GT255
is turned on. For example, when DN is 255, counter 202 moves the
output of decoder 204 to the gate line for GT255; thereby
connecting Vgamma voltage to line 102 which connects to the column
driver of the selected sub-pixel. Since the Vgamma voltage is the
maximum voltage put out by the Gamma Network 104, the maximum
voltage is placed on the gate of T1 in the selected sub-pixel. This
maximum voltage causes TFT T1 in the selected sub-pixel to supply
the current to OLED D1 for the brightest gray level for the
sub-pixel. The voltage value of Vgamma is determined by the design
of T1 and the designed top brightness of the sub-pixel. The methods
of doing such design work are well known in the display industry.
The table in FIG. 3B is an example of design voltages for Vgamma
and the taps on the voltage divider. For example, the design
voltage for Vgamma from FIG. 3B is 12 V. As a further example, if
the sub-pixel is scheduled by the image data to be black (off) then
DN 0 is sent to the gamma network 104 causing counter 202 to move
the output of decoder 204 to switch GT0 connecting Vtap0 to the
output line 102. The voltage value of Vtap0 from the table in FIG.
3B is 1.541 Volts, which when supplied to the gate of T1 through
the column driver for the selected sub-pixel causes the current
supplied to OLED D1 to be less than the threshold current for OLED
D1 and therefore, no light will be emitted from the sub-pixel for
the frame. The taps on the gamma network voltage divider 104 will
be between Vgamma and Vtap0 (12 Volts and 1.541 Volts,
respectively, in the example). As a further example, if DN 227 is
sent to gamma network 104, counter 202 will move the output of
decoder 204 to the gate line for switch GT227 connecting to the
aforesaid voltage divider 104 at a point between Vgamma and Vtap7.
The exact voltage connected through switch GT227 to output line
102, and thus, to the gate of TFT T1 in the selected sub-pixel will
be determined by the voltage drop from Vgamma to Vtap7, which from
the table in FIG. 3B is determined to be 12 Volts-10.729
Volts=1.271 Volts. There are 31 resistors (255-224=31) between
Vgamma and Vtap7; therefore, the voltage is dropped in 31 equal
decrements from Vgamma to Vtap, because all 31 resistors are of the
same value, which from the FIG. 3B is 7843 Ohms each. Each voltage
drop, therefore, is 1.271/31=0.041 volts. There are 28 resistors
(255-227) between the GT227 tap and the GT255 tap; therefore, the
voltage drop is 28.times.0.041=1.148 Volts. The exact voltage sent
to the selected sub-pixel through output line 102 and the column
driver to the gate of TFT T1 is 12 volts-1.148 Volts=10.852 Volts,
which is the T1 gate voltage designed to supply the required
current to OLED D1 to emit brightness corresponding to gray level
227. The other voltages at the various gray levels are calculated
in the same manner.
[0019] Referring back to FIG. 1, the OLED display 100 requires
regulated current in each sub-pixel to produce a desired brightness
from the pixel. Ideally, the TFTs T1 in each sub-pixel 120 should
be good current sources that deliver the same current for the same
gate voltage over the lifetime of the OLED display. Also each
current source TFT T1 in the active TFT matrix must deliver the
same current for the same data voltage stored in the storage
capacitor Cs in order that the display is uniform.
[0020] Note that there are two types of thin film semiconductors in
popular use in the active matrix display industry: amorphous
silicon (a-Si) and poly-silicon (p-Si). Emissive displays, such as
the active matrix OLED (AMOLED) displays, require high current and
stability not available in the a-Si TFTs and therefore typically
use p-Si for the TFTs T1, T2. a-Si is converted to p-Si by laser
annealing the a-Si to increase the crystal grain size and thus
convert a-Si to p-Si. The larger the crystal grain size, the faster
and more stable is the resulting semiconductor material.
Unfortunately the grain size produced in the laser anneal step is
not uniform due to a temperature spread in the laser beam. Thus,
uniform TFTs T1, T2 are very difficult to produce and thus the
current supplied by TFTs T1 in conventional OLED displays is often
non-uniform, resulting in non-uniform display brightness.
Non-uniform TFTs T1 throughout the OLED display causes "Mura" or
streaking in the OLED displays made with p-Si TFTs. In other words,
TFTs T1 may produce different OLED current due to its
non-uniformities from sub-pixel to sub-pixel, even if the same gate
voltage is applied to the TFTs T1. Therefore, it is necessary to
compensate for non-uniformities in the TFTs T1 by applying
corrected (compensated) T1 gate voltages that are different from
the intended gate voltage from the graphics board (not shown) to
the TFTs T1. This can be done by measuring the gray level (gate
voltage) versus current characteristics of the TFTs T1 for each
sub-pixel, and using such current measurement data to compensate
for the non-uniformities in TFTs T1 when driving the TFTs T1 with
the gate voltage 102 through the gamma network 104.
[0021] Another problem with AMOLED displays occurs due to aging of
the material in the OLEDs. As the OLED sub-pixels age with use,
OLEDs become less efficient in converting current to light, i.e.,
the efficiency of light emission of the OLEDs decreases. Thus, as
OLED current to light efficiency of the OLED material decreases
with use (age), light emitted from an OLED sub-pixel for a given DN
number also decreases, because the gamma network 104 in
conventional AMOLED does not compensate for the decreased
efficiency of light emission in the aged OLED sub-pixels. As a
result, the OLED display emits less light for display than desired
in response to a given DN. In addition, since the OLED sub-pixels
on various parts of the AMOLED display do not age (are not used)
equally in a uniform manner, OLED aging also causes non-uniformity
in the OLED display.
[0022] Thus, there is a need to solve problems associated with
aging of the OLED sub-pixels.
SUMMARY OF THE INVENTION
[0023] Embodiments of the present invention include methods of
determining the amount of compensation needed for reduced light
efficiency in aged sub-pixels of an active matrix organic
light-emitting diode (OLED) display, using a current ratio or a
voltage ratio pertaining to an aged sub-pixel relative to un-aged,
reference sub-pixels.
[0024] In one embodiment, the method comprises aging a plurality of
sections of sub-pixels of the active matrix OLED display, with the
sections including at least a first section including aged
sub-pixels and a second section including reference sub-pixels that
are not aged, applying a predetermined voltage across one of the
aged sub-pixels and at least one of the reference sub-pixels,
determining a first current through said one of the aged sub-pixels
and a second current through said at least one of the reference
sub-pixels, determining an age of said one of the aged sub-pixels
based on the first current relative to the second current,
determining light emission characteristics of said one of the aged
sub-pixels, and determining corrections to be made to digital
numbers indicative of desired brightness in said one of the aged
sub-pixels based on mappings between the determined age and the
determined light emission characteristics of said one of the aged
sub-pixels.
[0025] In another embodiment, the method comprises aging a
plurality of sections of sub-pixels of the active matrix OLED
display, with the sections including at least a first section
including aged sub-pixels and a second section including reference
sub-pixels that are not aged, determining a first voltage applied
to one of the aged sub-pixels to generate a predetermined reference
current through said one of the aged sub-pixels, determining a
second voltage applied to one or more of the reference sub-pixels
to generate the same predetermined reference current through said
one or more of the reference sub-pixels, determining an age of said
one of the aged sub-pixels based on the first voltage relative to
the second voltage, determining light emission characteristics of
said one of the aged sub-pixels, and determining corrections to be
made to digital numbers indicative of desired brightness in said
one of the aged sub-pixels based on mappings between the determined
age and the determined light emission characteristics of said one
of the aged sub-pixels.
[0026] According to the present invention, it is possible to
conveniently determine the age of an aged sub-pixel relative to an
un-aged reference sub-pixel using voltage ratios or current ratios,
and correlate such age measurement with the correction that needs
to be made to the DNs in order to compensate for reduced light
efficiency of the aged sub-pixels of the OLED display.
[0027] The features and advantages described in the specification
are not all inclusive and, in particular, many additional features
and advantages will be apparent to one of ordinary skill in the art
in view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and may not have been selected to delineate or
circumscribe the inventive subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The teachings of the embodiments of the present invention
can be readily understood by considering the following detailed
description in conjunction with the accompanying drawings.
[0029] FIG. 1 illustrates a conventional active matrix OLED
display.
[0030] FIG. 2 illustrates a conventional gamma network used with an
active matrix OLED display.
[0031] FIG. 3A illustrates a gamma curve showing the relationship
between the digital number (DN) and the OLED current.
[0032] FIG. 3B is a table showing example resistors, voltages and
currents for the gamma network in FIG. 2.
[0033] FIG. 4A illustrates an active matrix OLED display, according
to one embodiment of the present invention.
[0034] FIG. 4B illustrates the age correction circuit shown in FIG.
4A in more detail, according to one embodiment of the present
invention.
[0035] FIGS. 5A and 5B illustrate a sub-pixel of the AMOLED display
in more detail.
[0036] FIG. 6 illustrates how an AMOLED display is aged, according
to one embodiment of the present invention.
[0037] FIG. 7A illustrates a method of determining corrected
digital numbers (DNs) to use with aged sub-pixels of an AMOLED
display using current ratios, according to one embodiment of the
present invention.
[0038] FIG. 7B illustrates a method of determining corrected
digital numbers (DNs) to use with aged sub-pixels of an AMOLED
display using voltage ratios, according to one embodiment of the
present invention.
[0039] FIG. 8 illustrates the relationship between OLED brightness
and digital numbers (DNs) for different ages of the OLEDs,
according to one embodiment of the present invention.
[0040] FIG. 9A illustrates a method of determining the appropriate
age curve look-up table (LUT) to use for age compensation using
current ratios, according to one embodiment of the present
invention.
[0041] FIG. 9B illustrates a method of determining the appropriate
age curve look-up table (LUT) to use for age compensation using
voltage ratios, according to one embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0042] The Figures (FIG.) and the following description relate to
preferred embodiments of the present invention by way of
illustration only. It should be noted that from the following
discussion, alternative embodiments of the structures and methods
disclosed herein will be readily recognized as viable alternatives
that may be employed without departing from the principles of the
claimed invention.
[0043] Reference will now be made in detail to several embodiments
of the present invention(s), examples of which are illustrated in
the accompanying figures. It is noted that wherever practicable
similar or like reference numbers may be used in the figures and
may indicate similar or like functionality. The figures depict
embodiments of the present invention for purposes of illustration
only. One skilled in the art will readily recognize from the
following description that alternative embodiments of the
structures and methods illustrated herein may be employed without
departing from the principles of the invention described
herein.
[0044] FIG. 4A illustrates an active matrix OLED display according
to one embodiment of the present invention, and FIG. 4B illustrates
the age correction circuit shown in FIG. 4A in more detail
according to one embodiment of the present invention. FIGS. 4A and
4B will be explained together. Referring to FIG. 4A, the AMOLED
display 400 of FIG. 4A is substantially the same as the AMOLED
display 100 of FIG. 1, except that a calibration engine 402, a
selection look-up table (LUT) 404, and an age correction circuit
408 are added. The age correction circuit 408 receives the standard
DN 101, row timing data 110, and column timing data 106, and
generates a corrected DN 410 compensating for error introduced by
aging of the OLED sub-pixels for output to gamma network 104.
[0045] Referring to FIG. 4B, age correction circuit 408 includes
correction LUT 456, curve selector 458, age curve LUTs 460-1,
460-2, 460-3, . . . , 460-n, and adder (summing function) 470. Age
curve LUTs 460 store the DN level increase (or decrease) .DELTA.DN
relative to the standard DN 101 that is needed to force the aged
OLED sub-pixels to display the desired brightness as represented by
the standard DN 101. In other words, age curve LUTs 460 store
mappings from standard DN 101 to .DELTA.DN 472. Methods of
determining the age curve content to store in the age curve LUTs
460 are described below with reference to FIGS. 7A and 7B. Each
sub-pixel 120 (or pixel) is assigned to one of the age curve LUTs
460 for age correction. Correction LUT 456 stores the mapping
between the sub-pixel number and one of the age curve LUTs 460 to
use for that sub-pixel number, during normal operation.
[0046] Referring to both FIGS. 4A and 4B, during manufacturing or
testing of an AMOLED display, voltage ratios or current ratios from
the OLED sub-pixels 120 may be measured 414, as explained in more
detail below with reference to FIGS. 7A and 7B, to determine the
age of the OLED of the sub-pixel and obtain light emission
characteristics of aged sub-pixels for different ages of the
sub-pixels. Such determined light emission characteristics of the
aged sub-pixels for different ages may be stored in each of the age
curve LUTs 460 for each age, as mappings between a standard DN 101
and a correction (.DELTA.DN) 472 (increase or decrease) to the
standard DN 101 that needs to be made for that age of the
sub-pixel. Mappings between a particular age of an OLED sub-pixel
and a particular age curve LUT 460 to use for that age are stored
in selection LUT 404. The process of filling the content in the age
curve LUTs 460 and selection LUT 404 may be completed during
manufacturing or testing of the AMOLED display, before the AMOLED
displays are put in actual use.
[0047] Referring to both FIGS. 4A and 4B, after the OLED display
has been in actual use and during a calibration phase of the AMOLED
display, calibration engine 402 determines the age of the aged
sub-pixel 120 using voltage ratio or current ratio as explained in
more detail with reference to FIGS. 9A and 9B, and then determines
the age curve LUT 460 to use for that aged sub-pixel by looking up
the selection LUT 404. Then, calibration engine 402 updates 412
correction LUT 456 based on the determined age of the aged
sub-pixel, so that the particular aged sub-pixel being calibrated
is assigned to the proper age curve LUT 460 for that determined
age. Calibration phase can occur, for example, while the electronic
device (e.g., mobile phone) in which the OLED display is used is
not in normal operation (e.g., in charge mode of the mobile
phone).
[0048] In normal operation, the standard DN 101 for a sub-pixel 120
is corrected by the age correction circuit 408 to a corrected DN
value 410, which is input to the gamma network 104 to drive the T1
gate voltage 102. This is explained in more detail in FIG. 4B.
Correction LUT 456 receives row timing data 105 and column timing
data 106 that include the row and column numbers to be driven,
respectively, from timing controller 112, and determines which
pixel (sub-pixel) is to be driven by the graphics controller (not
shown). As explained above, correction LUT 456 stores mappings
between the sub-pixel numbers (identified by row number 105 and
column number 106) and the number of the assigned age curve LUT 460
to use for that sub-pixel, as a result of calibration of the aged
pixels by calibration engine 402 as explained above and below in
more detail with reference to FIGS. 9A and 9B. Correction LUT 456
receives the row number 105 and the column number 106 of the
sub-pixel of the OLED display that is currently being driven, and
selects and outputs the age curve LUT number 457 to use for that
sub-pixel. Curve selector 458 is essentially a decoder, and
receives the selected curve number 457 and selects the
corresponding one of the age curve LUTs 460-1, 460-2 . . . , 460-n
to be used based on the selected curve number 457. For example, the
selected age curve LUT number 457 may indicate that age curve LUT
No. 3 460-3 should be used for the sub-pixel currently being
driven, in which case curve selector 458 selects age curve LUT No.
3 (460-3).
[0049] Meanwhile, the standard DN 101 output from timing controller
112 is input to curve selector 458 and adder 470. The selected age
curve LUT no. 3 (460-3) selects the correction .DELTA.DN (increase
or decrease) needed to be made to the standard DN 101 to compensate
for aging of the OLED material of the OLED sub-pixel, based on the
received standard DN 101. The correction .DELTA.DN 472 is added to
the standard DN 101 by adder (summing function) 470 to generate the
corrected DN 410. The corrected DN 410 is one that has been
compensated for aging of the OLED sub-pixel, and is provided to
gamma network 104 to drive the T1 gate voltage 102 of the aged OLED
sub-pixel.
[0050] Note that in another embodiment, age curve LUTs 460 may
store mappings between the standard DN 101 representing the desired
pixel brightness and the actual corrected DN 410 that is required
to force the aged OLED sub-pixels corresponding to that particular
aged pixel to emit the desired brightness, rather than the
correction .DELTA.DN (increase or decrease) needed to be made to
the DN 101. In such an embodiment, no adder is needed since the age
curve LUTs 460 outputs the corrected DN 410 itself. However, in
such embodiment more memory space would be needed to store the
longer bits of the actual corrected DN 410.
[0051] The number of age curve LUTs needed for age compensation in
the OLED display depends on the desired age resolution of the OLED
display, i.e., the granularity of the age compensation desired. In
one embodiment, when the OLED light emission efficiency has
decreased to 50% of its un-aged efficiency, the OLED is deemed to
have reached the end of its life. Assuming a 6-bit system is used
to store the age curve LUT numbers, 50% divided by 64 (=2.sup.6)
results in 0.78% efficiency difference between adjacent age curves.
For an OLED material that has a half-life of 20,000 hours, there
would be an age curve spaced approximately every 312 hours
(=20,000/64). Each of the 64 age curve LUTs would be associated
with a particular age for which it contains DN correction data.
[0052] FIGS. 5A and 5B illustrate a sub-pixel of the AMOLED display
in more detail. As shown in FIG. 5A, TFT T1 and OLED D1 are
connected in series between supply voltages Vdd and Vss. The same
current Ioled flows though both TFT T1 and OLED D1. When TFT T1 is
biased in the saturation region, Id=k(Vgs-Vt).sup.2 (Equation 1)
holds, where Vgs is the voltage between the gate and source of TFT
T1, Vt is the threshold voltage of T1, Vds is the voltage from
drain to source of TFT 1, Id is the current through TFT T1, and k
is a proportionality constant reflecting electron mobility of TFT
T1. Thus, the magnitude of the current Ioled (current Id) when T1
is biased in the saturated region is controlled by the gate voltage
on TFT T1. When TFT T1 is biased in the linear region,
Id=2k[(Vgs-Vt)Vds-Vds.sup.2/2] (Equation 2) holds. If TFT T1 is
biased in the linear region and its gate voltage is fixed, the
current is controlled by its drain-source voltage Vd across T1. In
addition, Vtotal=Vdd-Vss (Equation 3) and Vtotal=Vds+Voled
(Equation 4), where Vtotal is the total voltage across TFT T1 and
OLED D1, Vdd is the power supply voltage, Vd is the voltage across
TFT T1, Voled is the voltage across OLED D1, and Vss is ground
voltage (typically 0 volt).
[0053] If TFT T1 is placed in the linear mode by connecting the
gate of TFT T1 to the cathode of OLED D1 as shown in FIG. 5B, the
current Ioled is a function of the Voled and Vtotal. But since
Ioled is also a function of Ioled, Ioled cannot be found by just
knowing Vtotal, which is the only voltage that can be directly
measured. Knowing the threshold voltage Vt and k of TFT T1, current
measurement of Ioled will allow the calculation of Vds from
Equation 2, which can then be subtracted from Vtotal to obtain
Voled. If a specific voltage Vtotal is applied to the sub-pixel
120, the sub-pixel circuit will settle to a current Ioled as a
function of Vdd, Vss. Therefore, if two sub-pixels have the same
Vdd and Vss and their gates are connected to the cathodes to put
the OLEDs D1 in linear mode, then the current Ioled in the two
sub-pixels should be identical, assuming that the TFTs T1 and OLED
D1s in the two sub-pixels are identical. The TFT T1s in the two
sub-pixels are assumed to be stable and both sub-pixels are assumed
to be at the same temperature. If one sub-pixel is aged but another
sub-pixel is not aged and identical Vdd and Vss are applied to both
the aged sub-pixel and the un-aged sub-pixel (referred to herein as
the "reference sub-pixel"), the current Ioled in the reference
sub-pixel will be different from the current Ioled in the aged
sub-pixel, i.e., the OLED current Ip in the aged sub-pixel will be
less than the OLED current Ir in the reference sub-pixel. Stated in
another way, larger Vtotal (Vdd-Vss) needs to be applied to the
aged sub-pixel than to the reference sub-pixel to obtain the same
current Ioled in the aged sub-pixel and the reference sub-pixel,
due to the aged OLED D1 in the aged sub-pixel. These properties may
be used to determine the age of a sub-pixel.
[0054] FIG. 6 illustrates how an AMOLED display is aged, according
to one embodiment of the present invention. For example, aging of
the AMOLED display is carried out as in FIG. 6 in the laboratory
during characterization of the OLED production process, in order to
determine the proper correction needed to be made to the DNs in the
AMOLED displays put into actual use and aged. The active area 600
of the AMOLED test display is divided into a plurality of sections
each of which is aged differently and at least one section with
reference pixels that are not aged. For example, active area 600
includes 16 sections 602, 604, . . . , 630 and a reference pixel
section 632. Each of the sixteen sections 602, 604, . . . , 632
contains thousands of pixels, and is aged by having current flow
through its sub-pixels for a predetermined period of time, but with
each section having different amounts of current flowing through
its sub-pixels in order to produce sixteen different rates of
aging. For example, section 602 is aged for 250 hours at a
predetermined current level, say IA. Section 604 is aged for 250
hours but at twice the predetermined current level (2IA) that
produces a two to one aging acceleration and thus is effectively
aged 500 hours. The current levels are increased in a similarly
manner to 3IA, 4IA, . . . , 16IA for sections 606, 608, . . . ,
632, respectively, until the sixteenth section 632 is aged at a 16
to 1 rate to produce a section of pixels that have an effective age
of 4000 hours. After aging is completed in this manner, the display
has pixels ranging from 250 hours to 4000 hours in effective age.
The reference pixels 632 remain un-aged.
[0055] FIG. 7A illustrates a method of determining corrected
digital numbers (DNs) to use with aged sub-pixels of an AMOLED
display using current ratios, according to one embodiment of the
present invention. According to the method of FIG. 7A, a
predetermined reference voltage is applied to the OLED sub-pixels
in differently aged sections of the aged OLED 600 (FIG. 6) and the
resulting current and light emission in the OLED sub-pixels are
measured. As the OLED display ages, current through the OLEDs will
decrease and the current to light efficiency will also decrease.
Therefore, the current decrease is a measure of decrease in OLED
efficiency, from which a correction to DN may be deduced. An
assumption in the method of FIG. 7A is that the efficiency change
in the OLED is due to aging and not some other ambient parameter,
which is true in many practical instances.
[0056] More specifically, at step 702 the sections of the OLED
panel are aged, for example, according to the method illustrated
with reference to FIG. 6. Then, at step 704 same supply voltages
Vdd and Vss (see FIGS. 5A and 5B) are applied to the aged
sub-pixels in one ages section (602, 604, . . . , or 630) and to
the reference sub-pixels (un-aged sub-pixels) in un-aged section
632, and in step 706 the currents through one or more of the aged
sub-pixels and the currents through one or more of the reference
sub-pixels are measured and averaged to determine the average
sub-pixel current (Ip) in the selected aged section (602, 604, . .
. , or 630) and the average sub-pixel current (Ir) in the un-aged
section 632. One way of measuring the sub-pixel current of an OLED
display is taught in U.S. patent application Ser. No. 11/710,462,
filed by Walter Edward Naugler, Jr., et al. on Feb. 22, 2007 and
entitled "Method and Apparatus for Managing and Uniformly
Maintaining Pixel Circuitry in a Flat Panel Display," which is
incorporated by reference herein. Another way of measuring
sub-pixel current of an OLED display is taught in U.S. patent
application Ser. No. 12/018,455 filed by Walter Edward Naugler,
Jr., et al. on Jan. 23, 2008 and entitled "Sub-Pixel Current
Measurement for OLED Display," which is incorporated by reference
herein. Other conventional methods of measuring the sub-pixel
current of an OLED display may be used with embodiments of the
present invention. Note that the current driving TFT T1 should be
separated from the operation of the OLED D1 when current is
measured, which can be accomplished by tying the gate of TFT T1 to
the supply voltage Vss that is also coupled to the cathode of OLED
D1 to place the TFT T1 in linear mode. Supply voltage Vdd is chosen
to be small enough not to cause local heating in the sub-pixels. In
measuring sub-pixel current in step 706, all other pixels are
turned off by applying a gate voltage 120 to the gates of the TFTs
T1 calculated to switch each sub-pixel off with minimum dark
current. One way of switching OLED sub-pixels off to achieve
minimum dark current is taught in U.S. patent application Ser. No.
12/033,527, filed by Walter Edward Naugler, Jr. on Feb. 19, 2008
and entitled "Minimizing Dark Current in OLED Display Using
Modified Gamma Network," which is incorporated by reference herein.
Other conventional methods of reducing dark current may be used
with embodiments of the present invention.
[0057] At step 708, the current ratio (Ip/Ir) corresponding to the
aged sub-pixel is determined. For fixed supply voltages Vdd and
Vss, the current ratio (Ip/Ir) will be less than 1 as the aged
sub-pixels have less efficiency. The amount of current ratio
(Ip/Ir) less than 1 indicates the age of the pixel. Since it is
known which section of the OLED panel the measured aged sub-pixel
belongs to, the determined current ratio (Ip/Ir) is a measure of
the effective age of the aged sub-pixel and the current ratio
(Ip/Ir) and the age can be mapped. Thus, at step 708 the selection
LUT 404 is also updated to reflect a proper mapping between the
effective age (represented by the current ratio (Ip/Ir)) of the
aged sub-pixel and an age curve LUT number corresponding to the
effective age represented by the current ratio. Current from the
aged sections and the current ratio (Ip/Ir) will steadily become
smaller as the current measurement moves from the 250 hour-aged
section 602 to the 4000 hour-aged section 632.
[0058] At step 710, light emission characteristics in the aged
sub-pixel are determined. Specifically, at step 710 the light
emission (brightness in candela) of the aged sub-pixel for given
DNs is measured for a particular age of the OLED represented as the
current ratio (Ip/Ir).
[0059] At step 712, such light emission characteristics are used to
determine the corrected digital number needed to achieve a
particular brightness of an aged sub-pixel. FIG. 8 illustrates the
relationship between OLED brightness and digital numbers (DNs) for
different ages of the OLEDs, according to one embodiment of the
present invention. For example, the three curves 852, 854, 856 show
the brightness vs. digital number relationship for three different
pixel ages A1, A2, and A3, respectively. The data for the graph in
FIG. 8 may be obtained from the age test using the test display
shown in FIG. 6, assuming that the laboratory test display in FIG.
6 is identical in design and production process as the OLED display
units sent into the field for actual customer usage. Since the test
display of FIG. 6 is also an actual display, the OLED display may
be turned on by supplying a DN gray level to the pixels using a
graphics board (not shown) and the pixel brightness may be
measured, in order to obtain the DN data on the x-axis and the
brightness data on the y-axis. The brightness of the pixels may be
measured in candelas using an optical photometer.
[0060] From the graph in FIG. 8 it is possible to determine the
digital number (DN) needed to achieve a certain brightness in the
OLED for different ages of the OLED sub-pixels. For example, curves
852, 854, 856 represent the relations between DN and achieved
brightness for sub-pixels aged A1, A2, A3, respectively, with A3
being the most aged, followed by A2, and A1 being the least aged.
In order to achieve a brightness of B1, sub-pixel aged A1 (curve
852) requires DN of 150, sub-pixel aged A2 (curve 854) requires DN
of 200, and sub-pixel aged A3 (curve 856) requires DN of
approximately 230. If sub-pixel aged A1 is the reference sub-pixel,
sub-pixel aged A2 requires DN correction (.DELTA.DN) of +50 for
standard DN of 150, and sub-pixel aged A3 requires DN correction
(.DELTA.DN) of +80 for standard DN. Thus, at step 712 such DN
correction data with respect to a standard DN 150 is also stored in
each of the age curve LUTs 460 corresponding to the age (A2, A3) of
the sub-pixel.
[0061] Steps 704, 706, . . . , 712 are repeated, moving from one
aged section (602, 604, . . . , 630) to another aged section (602,
604, . . . , 630) in step 716, until the last aged sub-pixel
section is reached in step 714 and the process ends 718. Note that
the method of FIG. 7A is most effective if (i) the TFTs in the
AMOLED display are stable, (ii) the reference pixels are stable and
remain in the initial state over the lifetime of the display, (iii)
the temperature of the OLED display is uniform during measurement
of the current, (iv) the test currents used do not appreciably
increase the temperature, (v) the test displays are from a stable
production process, and (vi) the gamma networks 104 in the test
display of FIG. 6 are same as those that would be included in OLED
displays that are put in actual use in the field.
[0062] FIG. 7B illustrates a method of determining corrected
digital numbers (DNs) to use with aged sub-pixels of an AMOLED
display using voltage ratios, according to one embodiment of the
present invention. According to the method of FIG. 7B, a
predetermined reference current is applied to the OLED sub-pixels
to differently aged sections of the aged OLED display 600 (FIG. 6)
and the voltage (Vtotal=Vdd-Vss in FIGS. 5A and 5B) across the OLED
sub-pixel and light emission in the OLED sub-pixels are measured.
In one embodiment, the OLED sub-pixel may be forced to have the
reference current flow using conventional feedback circuits (not
shown herein). If Vss is fixed (e.g., at ground), Vtotal can be
measured by measuring Vdd. As the OLED ages, the voltage
(Vtotal=Vdd-Vss) required to have the reference current flow
through the OLEDs will increase. Therefore, the voltage increase is
a measure of decrease in the OLED efficiency, from which a
correction to DN may be deduced. An assumption in the method of
FIG. 7B is that the efficiency change in the OLED is due to aging
and not some other ambient parameter, which is true in many
practical instances.
[0063] More specifically, at step 752 the sections of the OLED
panel are aged, for example, according to the method illustrated
with reference to FIG. 6. Then, at step 754 the average supply
voltage Vdd (referred to as Vr) (with Vss fixed) needed to force
the predetermined reference current in one or more of the reference
sub-pixels in the reference pixel section 632 is determined. Also,
at step 756, the average supply voltage Vdd (referred to as Vp)
(with Vss fixed) needed to force the predetermined reference
current in one or more of the aged sub-pixels in the aged pixel
section (602, 604, . . . , 630) is determined.
[0064] At step 758, the voltage ratio (Vp/Vr) corresponding to the
aged sub-pixels is determined. For fixed reference current and
fixed Vss, the voltage ratio (Vp/Vr) will be greater than 1 as the
aged sub-pixels have less efficiency. The amount of voltage ratio
(Vp/Vr) greater than 1 indicates the age of the pixel. Since it is
known which section of the OLED panel the measured aged sub-pixels
belong to, the determined voltage ratio (Vp/Vr) is a measure of the
effective age of the measured sub-pixels and the voltage ratio
(Vp/Vr) and the age can be mapped. Thus, at step 758 the selection
LUT 404 is also updated to reflect a proper mapping between the
effective age (represented by voltage ratio) of the aged sub-pixels
and an age curve LUT number corresponding to the effective age
represented by the voltage ratio. The voltage Vp needed for the
aged sections and the voltage ratio (Vp/Vr) will steadily become
larger as the voltage measurement moves from the 250 hour-aged
section 602 to the 4000 hour-aged section 632.
[0065] At step 760, light emission characteristics in the aged
sub-pixel are determined. Specifically, at step 760 light emission
(brightness in candela) of the aged sub-pixel for given DNs is
determined. At step 762, such light emission characteristics are
used to determine the corrected digital number needed to achieve a
particular brightness of an aged sub-pixel, similar to the
embodiment of FIG. 7A, and such DN correction data with respect to
a standard DN is also stored in each of the age curve LUTs 460
corresponding to the age of the sub-pixel.
[0066] The process of steps 754, 756, . . . , 762 are repeated,
moving from one aged section (602, 604, . . . , 630) to another
aged section (602, 604, . . . , 630) in step 766, until the last
aged sub-pixel section is reached in step 764 and the process ends
768. Note that the method of FIG. 7B is also most effective if (i)
the TFTs in the AMOLED display are stable, (ii) the reference
pixels are stable and remain in the initial state over the lifetime
of the display, (iii) the temperature of the OLED display is
uniform during measurement of the current, (iv) the test currents
used do not appreciably increase the temperature, (v) the test
displays are from a stable production process, and (vi) the gamma
networks 104 in the test display of FIG. 6 are same as those that
would be included in OLED displays that are put in actual use in
the field.
[0067] A possible advantage of using the voltage ratio embodiment
of FIG. 7B over the current ratio embodiment of FIG. 7A is that the
same current is forced through the reference pixels and aged
pixels. The change in the supply voltage in the voltage ratio
embodiment of FIG. 7B is caused only by an increase in the OLED
voltage, Voled (see FIGS. 5A and 5B). On the other hand, the
current change in the current ratio embodiment of FIG. 7A is caused
by changes in both the OLED voltage (Voled) and the OLED current
(Ioled), which may slightly reduce the accuracy of the current
ratio embodiment of FIG. 7A.
[0068] FIG. 9A illustrates a method of determining the appropriate
age curve look-up table (LUT) to use for age compensation using
current ratios, according to one embodiment of the present
invention. The method of FIG. 9A is used during calibration of the
AMOLED display to determine how aged the OLED sub-pixels are and
how to compensate for the reduced light efficiency of the aged OLED
sub-pixels. The method of FIG. 9A may be performed by the
calibration engine 402 (see FIG. 4A). The method of FIG. 9A is
carried out with respect to an aged AMOLED display that has been in
use for some time, and may be performed multiple times during the
life of the AMOLED display, for example, periodically, or during
inactive periods of the AMOLED display, etc. Note that the aged
AMOLED display used with the methods of FIGS. 9A and 9B is one that
has been in actual use and is separate from the test OLED panel 600
shown in FIG. 6 which was used to generate the age curve LUTs
according to the methods described in FIGS. 7A and 7B. However, in
one embodiment the actual panel in use may also include un-aged,
un-used reference pixels similar to the reference pixels 632 in
FIG. 6. Such reference pixels on the actual panel in use have
minimal aging and are expected to stay in their pristine original
state despite being accessed occasionally for calibration. In
another embodiment the actual panel in use does not include
un-aged, un-used reference pixels, but the methods of FIGS. 9A and
9B may use the youngest pixels in place of the reference pixels in
such other embodiment.
[0069] At step 904, the same supply voltages Vdd and Vss (see FIGS.
5A and 5B) are applied to one of the aged sub-pixels (e.g.,
sub-pixel at Row 1, Column 1) and to one of the reference
sub-pixels (un-aged sub-pixels), and at step 906 the current (Ip)
through the aged sub-pixel and the current (Ir) through the
reference sub-pixel are measured. Note that alternatively the
average current through many of the reference sub-pixels may be
measured and averaged, for use in place of the referenced current
(Ir) for one reference sub-pixel in step 906, in which case the
reference current does not need to be measured every time step 906
is performed. The average reference current may be used instead.
Reference current (Ir) herein may refer to the current in one
reference sub-pixel or the average reference current through a
plurality of reference sub-pixels.
[0070] At step 908, the current ratio (Ip/Ir) corresponding to the
aged sub-pixel is determined. As explained above, the determined
current ratio (Ip/Ir) is a measure of the effective age of the
measured sub-pixel. Thus, at step 910 calibration engine 402 looks
up selection LUT 404 to select the proper age curve LUT number
corresponding to the determined age of the aged sub-pixel based on
the current ratio (Ip/Ir). At step 911 calibration engine 402
updates (412 in FIG. 4A) correction LUT 456 in the age correction
circuit 408 to reflect the selected age curve LUT number for the
aged sub-pixel. That way, in normal operation, standard DNs 101 for
the aged sub-pixel will be corrected by the selected age curve LUT
460. The process of steps 904, 906, . . . , 911 are repeated,
moving from sub-pixel to sub-pixel in step 914, until the last aged
sub-pixel is reached in step 912 and the process ends 914.
[0071] FIG. 9B illustrates a method of determining the appropriate
age curve look-up table (LUT) to use for age compensation using
voltage ratios, according to one embodiment of the present
invention. The method of FIG. 9B may also be used during
calibration of the AMOLED display to determine how aged the OLED
sub-pixels are and how to compensate for the reduced light
efficiency of the aged OLED sub-pixels. The method of FIG. 9B may
be performed by the calibration engine 402 (see FIG. 4A). The
method of FIG. 9B is thus carried out with respect to an aged
AMOLED display that has been in use for some time, and may be
performed multiple times during the life of the AMOLED display, for
example, periodically, or during inactive periods of the AMOLED
display, etc.
[0072] At step 954, a reference current is forced through one of
the aged sub-pixels and the reference sub-pixels. At step 956 the
average supply voltage Vdd (referred to as Vr) (with Vss fixed)
needed to force the predetermined reference current in the
reference sub-pixels is determined. Also, at step 956, the supply
voltage Vdd (referred to as Vp) (with Vss fixed) needed to force
the predetermined reference current in one of the aged sub-pixels
(e.g., sub-pixel at Row 1, Column 1) is determined. At step 958,
the voltage ratio (Vp/Vr) corresponding to the aged sub-pixel is
determined. As explained above, the determined voltage ratio
(Vp/Vr) is also a measure of the effective age of the measured
sub-pixel. Thus, at step 960 calibration engine 402 looks up
selection LUT 404 to select the proper age curve LUT number
corresponding to the determined age of the aged sub-pixel based on
the voltage ratio (Vp/Vr). At step 961 calibration engine 402
updates (412 in FIG. 4A) correction LUT 456 in the age correction
circuit 408 to reflect the selected age curve LUT number for the
aged sub-pixel. That way, in normal operation, standard DNs 101 for
the aged sub-pixel will be corrected by the selected age curve LUT
460. The process of steps 954, 956, . . . , 961 are repeated,
moving from sub-pixel to sub-pixel in step 964, until the last aged
sub-pixel is reached in step 962 and the process ends 964.
[0073] According to the present invention, it is possible to
conveniently determine the age of an aged sub-pixel relative to
un-aged reference sub-pixels using voltage ratios or current
ratios, and correlate such age measurement with the correction that
needs to be made to the DNs in order to compensate for reduced
light efficiency of the aged sub-pixels of the OLED display.
[0074] Upon reading this disclosure, those of skill in the art will
appreciate still additional alternative structural and functional
designs for correcting digital numbers in order to compensate for
reduced light efficiency of the aged sub-pixels of the OLED
display. For example, although various embodiments of the present
invention are illustrated as using voltage ratios or current
ratios, the age of the sub-pixels do not necessarily have to be
determined using strictly ratios, and any comparison of the current
or voltage in the aged sub-pixels relative to the current or
voltage in un-aged reference sub-pixels may be used. For instance,
differences rather than ratios may be used. Thus, while particular
embodiments and applications of the present invention have been
illustrated and described, it is to understood that the invention
is not limited to the precise construction and components disclosed
herein and that various modifications, changes and variations which
will be apparent to those skilled in the art may be made in the
arrangement, operation and details of the method and apparatus of
the present invention disclosed herein without departing from the
spirit and scope of the invention as defined in the appended
claims.
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